The Many-Worlds Interpretation of QM

[Maui]

Quote by Fredrik

The main assumption of the CI is that state vectors can be identified with physical systems, i.e. that each state vector describes all the properties of the system it represents. Let's label that assumption (1). I said that if we add this on top of QM, we get a contradiction, but that's not quite right. What we get is many worlds.

Why would there be a contradiction?

lol Occam's razor is a guideline not a law of nature! Anyway I don't really see how MWI goes against it at all...

I never stated or implied it was a law.

Would you like to elaborate on why you think so?

As William of Ockham once said - "Pluralitas non est ponenda sine neccesitate" or "plurality should not be posited without necessity."

If you need the generation of 100 billion different universes just to justify the eating of a seed by a mouse, the theory is as wasteful as a human being can ever imagine. Nothing could ever be more wasteful and uneconomical and it also leads to quantum immortality which has never been observed(in this universe).

 

[Fredrik]

Why would there be a contradiction?

I think you stopped reading too soon or something. The next thing I said is "but that's not quite right. What we get is many worlds". The statement about the contradiction (in QM+(1)) was a reference to an incorrect statement I had made earlier in that thread. The contradiction is in QM+(1)+(2), as I explained.

As William of Ockham once said - "Pluralitas non est ponenda sine neccesitate" or "plurality should not be posited without necessity."

But what really is "plurality"? An example that's often brought up in this context is the information required to specify a single integer vs. the information required to specify all the integers. There's no upper bound on how much information can be required to specify an integer, but you can specify all the integers with a simple recursive statement. The point of this is that it's not always the alternative that involves the larger number of "somethings" that's more complicated.

If you need the generation of 100 billion different universes just to justify the eating of a seed by a mouse, the theory is as wasteful as a human being can ever imagine. Nothing could ever be more wasteful and uneconomical and it also leads to quantum immortality which has never been observed(in this universe).

There's nothing wasteful about it. QM consists of a few simple statements about a simple mathematical structure, and the MWI is essentially just the assumption that QM describes reality.

It would be at least as valid to argue that the MWI is favored by Occam's razor, because the alternative is that either there's a completely different piece of mathematics that describes what actually happens, or reality isn't described by mathematics at all. Both of these options can be considered unnecessary complications.

I'm not saying that we should believe in the MWI because of Occam or any other reason. I don't think Occam is of any help here.

I consider the quantum immortality argument to be nonsense. I don't believe it makes sense to assign a probability to the possibility that "you will survive" in that scenario with that very unorthodox definition of "you". And even if I'm completely wrong about that, the MWI doesn't predict that quantum immortality will be observed.

 

[1977ub]

Is MWI falsifiable? How is it better than MMRNG (many mysterious random number generators) ? What's the best answer to "where does all the *new* matter /energy come from" ? Thank you.

 

[Fredrik]

Is MWI falsifiable?

It's not. That's why it's called an interpretation, and not a theory.

How is it better than MMRNG (many mysterious random number generators) ?

How is that an interpretation of QM?

The MWI is simply the assumption that QM describes the universe.

What's the best answer to "where does all the *new* matter /energy come from" ? Thank you.

There's no new matter. (I don't care what that old "Everett FAQ" says). The worlds are "aspects" of the properties of a single physical system.

 

[1977ub]

It's not. That's why it's called an interpretation, and not a theory.

Interesting. Can you list some other ideas in physics on this same level - unfalsifiable interpretations?

There's no new matter. (I don't care what that old "Everett FAQ" says). The worlds are "aspects" of the properties of a single physical system.

Do these "properties" have different subjective experiences & lives? (in this "interpretation").

 

[Fredrik]

Interesting. Can you list some other ideas in physics on this same level - unfalsifiable interpretations?

All of the interpretations of QM.

If you don't want to make unfalsifiable assumptions, you need to focus on the fact that the only thing we know for sure about QM is that it's an assignment of probabilities to measurement results that agrees very well with experiments.

There is some room for interpretation of some classical theories as well. I think both SR and GR have, as an alternative to the standard geometrical stuff, an interpretation that says that the results of experiments and observations aren't caused by the geometry of spacetime, but by changes to the properties of the measuring devices and the objects on which we do measurements. I don't know this stuff well enough to elaborate. Such an interpretation of GR was mentioned in "Black holes and time warps: Einstein's outrageous legacy" by Kip Thorne.

Do these "properties" have different subjective experiences & lives? (in this "interpretation").

There's nothing subjective about the properties of that single physical system (here called "the universe", lacking a better word). But if we choose a decomposition of the universe into subsystems (e.g this piece of matter + everything else), then we are able to interpret what's happening to the universe as a description of what's happening in many different worlds.

I don't think I'd be able to elaborate much more than this. I don't think anyone has ever written anything really good about these things.

 

[bohm2]

All of the interpretations of QM.

GRW is falsifiable as it makes slightly different predictions than QM. So, in principle, such theories can be tested against the standard one.

 

[1977ub]

There's nothing subjective about the properties of that single physical system (here called "the universe", lacking a better word). But if we choose a decomposition of the universe into subsystems (e.g this piece of matter + everything else), then we are able to interpret what's happening to the universe as a description of what's happening in many different worlds.

I meant in the sense of observers possessing subjectivity. We have a cat who experiences being alive in one infinity of universes and dying ones in a different infinity. Branching off from the one problematic measurement, all the infinities being experienced each in its own classical way by different sentient observers - this is what I was thinking of.

 

[Nugatory]

GRW is falsifiable as it makes slightly different predictions than QM. So, in principle, such theories can be tested against the standard one.

Yes, it is true that some interpretations have been successfully falsified. Another example is that Bell-type experiments have pretty much excluded all interpretations that consider QM to be a statistical description of local hidden variables.

Of course if an an interpretation is falsified, we no longer consider it a valid interpretation... So every once in a while a proposed interpretation drops by the wayside. But this doesn't change Fredrik's basic point, which is that there a number of interpretations that are not currently falsifiable, MWI is one of them, and the only thing we can verify is that the statistical predictions of QM agree with experiment.

(It is, of course, not a coincidence that Bell's argument is statistical in nature).

 

[bohm2]

Of course if an an interpretation is falsified, we no longer consider it a valid interpretation... So every once in a while a proposed interpretation drops by the wayside. But this doesn't change Fredrik's basic point, which is that there a number of interpretations that are not currently falsifiable, MWI is one of them, and the only thing we can verify is that the statistical predictions of QM agree with experiment.

(It is, of course, not a coincidence that Bell's argument is statistical in nature).

I agree about MWI, but he argued that all of the interpretations of QM are unfalsifiable. GRW can be considered an alternative theory not just an interpretation, I think, because it is theoretically testable.

 

[Maui]

I think you stopped reading too soon or something. The next thing I said is "but that's not quite right. What we get is many worlds". The statement about the contradiction (in QM+(1)) was a reference to an incorrect statement I had made earlier in that thread. The contradiction is in QM+(1)+(2), as I explained.

Your (1) assumption is almost certainly wrong as has been found by the quantum eraser experiment, some variations of the double slit, recent experiments on the HUP utilizing weak measurements, etc. As it turns out, it's the availability of the which-way information that destroys the interference pattern and not interactions with detector's photons or other obstacles. As far as i can see, it thus invalidates the MWI as well, which requires that interactions between wavefunctions create particle-like detections and classicality in an endless world splitting.

 

[Nugatory]

I agree about MWI, but he argued that all of the interpretations of QM are unfalsifiable. GRW can be considered an alternative theory not just an interpretation, I think, because it is theoretically testable.

I think we're all largely in agreement here... Once an interpretation becomes falsifiable we reclassify it as a theory not an interpretation (as suggested by your comfort with word "because" above) so it's a tautology to say that interpretations are not falsifiable.

At the risk of putting my words in Fredrik's mouth, I expect that he would consider a falsified proposition to be a dead theory not a live interpretation.

 

[Fredrik]

Your (1) assumption is almost certainly wrong as has been found by the quantum eraser experiment, some variations of the double slit, recent experiments on the HUP utilizing weak measurements, etc.

This can't be right, since (1) doesn't change any of the theory's predictions.

 

[Fredrik]

I think we're all largely in agreement here... Once an interpretation becomes falsifiable we reclassify it as a theory not an interpretation (as suggested by your comfort with word "because" above) so it's a tautology to say that interpretations are not falsifiable.

At the risk of putting my words in Fredrik's mouth, I expect that he would consider a falsified proposition to be a dead theory not a live interpretation.

Yes, this is what I'm thinking.

 

[bhobba]

Is MWI falsifiable? How is it better than MMRNG (many mysterious random number generators) ? What's the best answer to "where does all the *new* matter /energy come from" ? Thank you.

MWI is as falsifiable to the extent QM is falsifiable. There however is no way to experimentally tell it from any other of the myriad of interpretations.

MWI does not postulate the creation of new matter and energy in the sense we generally think of it - it is preserved in any world it splits into - it simply takes literally the existence of a quantum state and has it as very real with an objective existence and doesn't have quantum state collapse as the result of an observation.

It is in fact a very elegant and mathematically beautiful interpretation - its just that many people like me can't stomach what it postulates and think its way too weird and not at all required. But that is a belief, an opinion, I have about how nature is - science is not about beliefs - its about correspondence with experiment. Just because I think it weird does not mean its not correct - it may well be.

Thanks
Bill

 

[1977ub]

If one subscribes comfortably to MWI, could this sap one's motivation for finding some less... "infinite" explanation of QM?

 

[bhobba]

If one subscribes comfortably to MWI, could this sap one's motivation for finding some less... "infinite" explanation of QM?

Well let's see - what you are asking is if someone believes in something some think is too weird to be true could this sap ones motivation for finding other interpretations others think is less weird? Maybe they are scientist enough to not let such things get in the way but who knows. If such things interest you maybe psychology would be a better subject to study.

Thanks
Bill

 

[bhobba]

Interesting. Can you list some other ideas in physics on this same level - unfalsifiable interpretations?

Yea - Lorentz Ether Theory (LET) vs Special Relativity (SR).

Your choice depends on how you think the world works but most reject LET because of its unobservable ether. Also when its extended to Quantum Field Theory you require extra ad-hoc assumptions. In fact the ether is an ad-hoc assumption like if you were to postulate forces cause unobservable ghosts to move objects rather than forces themselves - you can't prove it wrong - but most reject it as - well - silly - which it is.

There are probably others about as well.

Such things have more of a psychological origin than physical.

I have answered a number of your queries and they all are more along the lines of philosophy than physics - maybe a philosophy forum would be a bit more appropriate for what interests you. I have zero problems with answering them but I am a bit of an anti philosophy type sort of like Feynman was and the kind of answers I give may not satisfy those of a more philosophical bent.

Thanks
Bill

 

[bhobba]

GRW is falsifiable as it makes slightly different predictions than QM. So, in principle, such theories can be tested against the standard one.

Yea that's true - some of what are called interpretations are in fact different theories (eg primary state diffusion as well as the GRW you mention) - but sometimes physicists are a bit lax in terminology.

Thanks
Bill

 

[bhobba]

As far as i can see, it thus invalidates the MWI as well, which requires that interactions between wavefunctions create particle-like detections and classicality in an endless world splitting.

It can't invalidate it since MWI is simply bog standard QM with the measurement postulate removed - instead of the wavefunction changing it simply 'splits'. To be more specific MWI includes decoherence (in fact it was the first decoherence based interpretation - by which is meant it includes the phenomena of decoherence in its foundations) so a superposition is transformed into an improper mixture. Instead of an observation selecting one of the states of the mixture they all exist simultaneously but in different worlds. Extremely neat way of resolving the problem - very neat - but can you stomach its implication - that's the issue.

Actually in discussing it in this thread I can see why its so appealing to its supporters - it really is elegant.

Thanks
Bill

 

[Nugatory]

If one subscribes comfortably to MWI, could this sap one's motivation for finding some less... "infinite" explanation of QM?

Because interpretations are not falsifiable, you can subscribe to whichever one you find most satisfying, and as long as it is satisfying to you there's no particular motivation to go looking for another one. But is this a problem? We're talking about an individual's aesthetic preference, which is why two different people speaking of MWI can say

it really is elegant.

and

If you need the generation of 100 billion different universes just to justify the eating of a seed by a mouse, the theory is as wasteful as a human being can ever imagine. Nothing could ever be more wasteful and uneconomical...

I see no reason to argue with either position. It's discussions like this one that send me back into my tiny cave where can I huddle in the darkness and clutch my favorite interpretation ("shut up and calculate") to my breast

 

[bohm2]

Another model that can be considered an alternative theory to QM is nonequilibruim Bohmian mechanics. But its hopes lie in finding such nonequilibrium violations of QM in cosmic microwave background, etc. Should such violations be discovered that would be evidence for Bohmian mechanics as a new post-quantum theory; however, without any such discovery Bohmiam would just be an interpretation like many of the others and arguably less so according to Valentini. Of course, there's quite a few "if"s:

Should inflation be very firmly established, and should it be found that the predictions of quantum theory continue to hold well at all accessible lengthscales during the inflationary era, then this would constitute considerable evidence against the hypothesis of quantum nonequilibrium at the big bang (though of course, nonequilibrium from an earlier era might simply have not survived into the inflationary phase). Furthermore, it would rather undermine the view that quantum theory is merely an effective description of an equilibrium state. In principle, one could still believe that hidden variables exist, and that the hidden variables distribution is restricted to quantum equilibrium even at the shortest distances and earliest times. But in the complete absence of nonequilibrium, the detailed behaviour of the hidden variables (such as the precise form of the trajectories in de Broglie-Bohm theory) would be forever untestable. While exact equilibrium always and everywhere may constitute a logically possible world, from a general scientific point of view it seems unacceptable, and the complete ruling out of quantum nonequilibrium by experiment would suggest that hidden-variables theories should be abandoned. On the other hand, a positive detection of quantum nonequilibrium phenomena in the early universe (or indeed elsewhere) would be of fundamental interest, opening up a new and deeper level of nature to experimental investigation.

Inflationary Cosmology as a Probe of Primordial Quantum Mechanics
http://lanl.arxiv.org/pdf/0805.0163.pdf

 

[bhobba]

It's discussions like this one that send me back into my tiny cave where can I huddle in the darkness and clutch my favorite interpretation ("shut up and calculate") to my breast

Otherwise known as the Minimum Statistical Interpretation (MSI)- but even that has slight variations - but its basically what I hold to as well.

I find it a bit of an unnerving experience discussing interpretations I don't hold to - but still interesting.

Thanks
Bill

 

[bhobba]

Another model that can be considered an alternative theory to QM is nonequilibruim Bohmian mechanics.

Yes work is being done all the time on what is usually thought to be an interpretation but may be a different theory.

A while ago there was a lot of discussion on if Bohmian Mechanics was a different theory and some thought it was with papers written about how it could be tested. In fact someone even did an experiment that supposedly falsified it.

I originally thought that was the case but it wasn't until I had a chance to discuss it with some on this forum I found the error - it had to do with a mistaken use of the Dirac Delta function. But it does show how difficult this stuff actually is.

Thanks
Bill

 

[1977ub]

Yea - Lorentz Ether Theory (LET) vs Special Relativity (SR).

Your choice depends on how you think the world works but most reject LET because of its unobservable ether. Also when its extended to Quantum Field Theory you require extra ad-hoc assumptions. In fact the ether is an ad-hoc assumption like if you were to postulate forces cause unobservable ghosts to move objects rather than forces themselves - you can't prove it wrong - but most reject it as - well - silly - which it is.

There are probably others about as well.

Such things have more of a psychological origin than physical.

I have answered a number of your queries and they all are more along the lines of philosophy than physics - maybe a philosophy forum would be a bit more appropriate for what interests you. I have zero problems with answering them but I am a bit of an anti philosophy type sort of like Feynman was and the kind of answers I give may not satisfy those of a more philosophical bent.

Thanks
Bill

MWI drives me to it.

 

[bhobba]

MWI drives me to it.

No problemo - the only reason I mention it if you have a philosophical bent the answers from a guy like me, and I suspect some others who regularly post around here, may not satisfy.

Thanks
Bill

 

[Maui]

This can't be right, since (1) doesn't change any of the theory's predictions.

Different interpretations propose different routes to the same predictions. If the route is found to be wrong, so is the interpretation. The (1) assumption seems invalid in light of the experiments i cited earlier.

 

[Maui]

It can't invalidate it since MWI is simply bog standard QM with the measurement postulate removed - instead of the wavefunction changing it simply 'splits'. Thanks
Bill

That's the whole point - the experiments i mentioned in post 46, namely

the quantum eraser experiment, some variations of the double slit, recent experiments on the HUP utilizing weak measurements, etc.

all require a measurement postulate based on the which-way information being available or not.

 

[bhobba]

That's the whole point - the experiments i mentioned in post 46, namely
all require a measurement postulate based on the which-way information being available or not.

Since they conform to bog standard QM they can't falsify it. Its like the claims that Bohmian mechanics had been falsified - even I got caught up in it. But it can't - BM is deliberately concocted to be exactly the same in terms of predictions as bog standard QM.

I am afraid if you are to convince me, and I suspect others that post around here, it has been falsified you will need to detail exactly in what way when decoherence occurs that one outcome is not selected from the ensemble (ie mixture), but rather all occur simultaneously in different worlds, in anyway leads to an experimentally different outcome in any of those worlds that is different to if it was selected as the only one. Since there is no way of telling the difference as an observer in one of those worlds there is no way to tell the difference.

What you have posted from my perspective doesn't tell me how its falsified - a lot more detail is required for me to take it seriously.

Thanks
Bill

 

[Maui]

Since they conform to bog standard QM they can't falsify it. Its like the claims that Bohmian mechanics had been falsified - even I got caught up in it. But it can't - BM is deliberately concocted to be exactly the same in terms of predictions as bog standard QM.

The MWI conforms to the standard QM via a different "route". That the interpretations reach the same target(predictions) doesn't mean that the hypothetical route(s) of the MWI or other interpretaions is right. The mathematical formalism says nothing about this route.

I am afraid if you are to convince me, and I suspect others that post around here, it has been falsified you will need to detail exactly in what way when decoherence occurs that one outcome is not selected from the ensemble (ie mixture), but rather all occur simultaneously in different worlds, in anyway leads to an experimentally different outcome in any of those worlds that is different to if it was selected as the only one. Since there is no way of telling the difference as an observer in one of those worlds there is no way to tell the difference.

No, again they all lead to the same predictions, but the way to the predictions is different for the different interpretations. There is nothing(AFAIK) that prevents decoherence to take place when information about an otherwise contextual system becomes available(you isolate it from the environemnt - and no information can be extracted and the system returns to its quantum state - sometimes measured and verfied through weak measurements). When you don't isolate it(and information is constantly available) the system is mostly in its classical, particle-like state(coherence is destriyed).

I have no agenda or point to prove. As many of the others here i just want to know better what is going on, that's all.

Its like the claims that Bohmian mechanics had been falsified - even I got caught up in it. But it can't - BM is deliberately concocted to be exactly the same in terms of predictions as bog standard QM.

If you take seriously the weak measurements experiments that 'show' superpositions of states, i am afraid the BI has to go as well.

 

[bhobba]

No, again they all lead to the same predictions, but the way to the predictions is different for the different interpretations.

If they all lead to the same predictions how can it be falsified compared to another theory that has the same predictions?

Specifically take the MSI (Minimal Statistical Interpretation) of the experiments you cite - since this is bog standard QM it must predict the results of those experiments - if it didn't that would be big news because QM would have been falsified. Now the only difference between MSI and MWI is that the wave function did not collapse - instead it split into a number of worlds with each world experiencing a different possible outcome. To an observer in any of those worlds there is no way - none - zero - zilch - for them to tell the difference between MWI and the MSI. Because of that since the experiments you cite all conform to the MSI it must conform to MWI - there is no way it can't - its impossible.

To put it another way - yes the MWI is a different route than the MSI but the MWI has been deliberately concocted to be indistinguishable from the MSI.

Just to be 100% clear take for example the theory that forces do not cause objects to move, but rather it causes ghosts to move the objects. Its obvious this theory is a different route to objects moving but its just as obvious there is no way to tell the difference.

Thanks
Bill

 

[Fredrik]

Different interpretations propose different routes to the same predictions. If the route is found to be wrong, so is the interpretation. The (1) assumption seems invalid in light of the experiments i cited earlier.

That first sentence is only true about interpretations that change the mathematics of the theory. My (1) is a non-mathematical assumption that's added on top of QM. So every bit of QM remains intact. (1) is just a guess about what it all means.

 

[Quantumental]

This thread is missing discussion on the two most recent and relevant papers on the Many Worlds interpretation.

Here is a interesting paper showing that Many Worlds is incoherent: http://philsci-archive.pitt.edu/9542/1/Decoherence_Archive.pdf

Here is another one showing that the preferred basis problem has not been solved:
http://arxiv.org/abs/1210.8447

 

[Maui]

If they all lead to the same predictions how can it be falsified compared to another theory that has the same predictions?

Specifically, there are experiments that say specifically that it's the which-way information that causes wavefunctions to collapse, not interactions as the MWI requires. MWI requires something(interaction between wavefunctions to give the impression of 'particles') and that is not what the experiments i listed earlier show.

Specifically take the MSI (Minimal Statistical Interpretation) of the experiments you cite - since this is bog standard QM it must predict the results of those experiments - if it didn't that would be big news because QM would have been falsified.

You obviously think that the role of an interpretion is to make correct predictions, whereas its role is to explain the behavior of the respective system while arriving at the same predictions as the formalism. I have no idea why this point isn't obvious.

 

[Maui]

That first sentence is only true about interpretations that change the mathematics of the theory.

Why? The MWI and the copenhagen interpretation both propose very different routes to classicality. How do they change the formalism?

My (1) is a non-mathematical assumption that's added on top of QM. So every bit of QM remains intact. (1) is just a guess about what it all means.

QM is certainly intact but the assumption is hard to support for the experiments cited earlier.